Serum represents the liquid portion of the blood which remains following the clotting of the formed blood elements and blood coagulation factors. The serum contains various blood proteins, sugars, fats, enzymes and charged particles (i.e., electrolytes) which are necessary for normal metabolic processes. Devoid from the serum are the elements consumed during the clotting process namely red blood cells, white blood cells, platelets and the blood coagulation factors. Serum, obtained by centrifugation of clotted blood, is used to conduct chemical analyses which are valuable tests used to diagnose and monitor muscle and organ function, metabolic balances, and basic physiologic functions. By removing the substances consumed during clotting, a better substrate upon which to perform chemical analyses is produced. Thus, serum is the preferential test substrate on which to perform analytical tests such as enzyme, electrolyte, protein and glucose assays since the interference of unwanted substances has been removed. These analytical tests are routinely performed using automated chemistry analyzers such as the Hitachi 717, Technicon SMAC, and Ciba-Corning 5500 Express.
In the past, the production of serum from whole blood has been a passive process in which freshly collected blood is added to a glass test tube and allowed to clot. Blood, once removed from the body, has a natural tendency to clot and its exposure to a surface such as glass promotes clotting in a more efficient manner. Contact with a glass surface causes the activation of coagulation factors which interact in a mechanism commonly referred to as the coagulation cascade. In this process, an inactive coagulation factor is chemically converted to an active enzyme which subsequently converts yet another inactive precursor. The coagulation factors are identified by Roman Numerals; Factors I-XIII. The end result of the coagulation cascade is a conversion of the soluble plasma protein fibrinogen, to an insoluble protein, fibrin, whereby the fibrin clot entraps the white cells, red cells, and platelets forming a solid gelatinous mass. Substances not consumed in the process remain free of the gelatinous mass and are found in the liquid matrix. It is this liquid portion which serves as the best test substrate upon which to perform serum chemistry analyses.
The passive clotting process described above has several inadequacies. Blood from normal healthy individuals will routinely clot in approximately 6 to 10 minutes in a glass test tube. However, blood from sick individuals who may have deficiencies of coagulation proteins or from patients who are receiving anticoagulation therapy (i.e., oral anticoagulants or heparin) may require extensive time to clot (i.e., 2-8 hours). Typically highly heparinized blood is between 2-5 units per milliliter and possibly higher. Consequently, in the past, there has been a delay associated with the obtaining of blood specimens and the performance of the analytical tests, thereby affecting the ability of the clinician to quickly provide optimal patient care. In addition, the blood from individuals with deficiencies of coagulation proteins or patients receiving anticoagulation therapy may never form a complete and adequate fibrin mass. Incomplete clotting in heparinized blood specimens results in a poor quality serum substrate upon which to perform the chemical test. Furthermore, heparin anticoagulated blood which may not clot initially may actually begin to clot once placed in the chemistry analyzer, thereby clogging the system and causing an instrument shutdown.
In order to improve the clot forming process, laboratorians have routinely added the clot promoting agent thrombin to the blood specimen. Thrombin actively converts fibrinogen to fibrin, thereby forming the clot more efficiently then the slower glass-activated clotting process. Although thrombin may improve clotting in normal blood, it's effects on heparinized blood are minimal since heparin acts as an inhibitor of thrombin. As a result, collection of heparinized blood in these thrombin containing serum test tubes provides little improvement of the serum specimens, both in quality as well as in the time required to prepare the specimens, as compared to the plain glass test tube. Thrombin containing test tubes are available from Becton-Dickinson and Company, VACUTAINER Systems, of Rutherford, N.J. 07070. Though it is possible to overcome the anticoagulant effect of heparin with very large amounts of thrombin, the fibrin mass so formed is often an incomplete one. Consequently, following the centrifugation of an apparently clotted specimen, delayed clotting occurs in the serum supernatant rendering it of little use in analytical tests. Moreover, such an excessive thrombin quantity will interfere with the proper performance of the chemical analyses, thereby altering the test results. Furthermore, excessive procoagulants such as thrombin can cause some red blood cell hemolysis, whereby the red blood cell bursts open contaminating the serum specimen with intracellular particles. The above stated problems relating to the rapid production of a high quality serum specimen from heparinized patients on which to perform chemical analyses are significant since many patients receiving heparin anticoagulation therapy require repetitive serum chemistry analyses.